This invention relates to a ferritic stainless steel and its welded pipe used in the exhaust gas passage components of an automobile, typically in the exhaust manifold, catalytic converter case (cylindrical casing), front pipe and center pipe, and to automobile exhaust gas passage components utilizing the ferritic stainless steel and welded steel pipe.
The exhaust gas passage components of an automobile, such as the exhaust manifold, catalytic converter case, front pipe and center pipe, are required to be excellent in high-temperature oxidation resistance and high-temperature strength in the high-temperature region exceeding 700° C. As a material having such heat resistance, Patent Documents 1 and 2 teach ferritic stainless steels added with about 1 to 2 mass % of Cu. The Cu in the steel precipitates as Cu phase under heating to improve the high-temperature strength and thermal fatigue property of the steel.
Most of the aforesaid automobile exhaust gas passage components are produced by shaping welded steel pipes. Owing to the increasing number of units installed in the engine compartment in recent years, the amount of space available for installation of exhaust gas passage components has continued to decrease. This has led to many exhaust gas passage components being manufactured in complex shapes by special processing. The welded steel pipes used in exhaust gas passage components are therefore required to have even better formability than heretofore.
Regarding a technique for improving the formability of welded steel pipe made of ferritic stainless steel, Patent Document 3 teaches that trace addition of Al or Ti enhances the toughness and secondary workability of the weld. However, a study carried out by the inventors showed that trace addition of Al or Ti to ferritic stainless steel improved in high-temperature strength by inclusion of 1 to 2% Cu as mentioned above does not readily ensure sufficient toughness of a steel pipe produced by high-frequency welding. Moreover, sufficient toughness is even harder to achieve in a component such as a catalytic converter case because the component is manufactured by subjecting a steel pipe that has been TIG welded or laser welded to very severe compressive working (pressing or spinning). In other words, it was found that a welded steel pipe made of a ferritic stainless steel containing around 1 to 2% Cu cannot be adequately improved in toughness merely by trace addition of Al or Ti as taught by Patent Document 3.
In addition, the weld toughness of a high-frequency welded pipe is particularly easily affected by the pipe-making conditions determined by the amount of upset and heat input. In a ferritic stainless steel containing 1 to 2% Cu, the difficulty of consistently securing good toughness becomes even greater when the pipe-making conditions deviate from the optimum conditions.
Patent Document 1: WO 03/004714
Patent Document 2: JP 2006-117985A
Patent Document 3: JP 2005-264269A